The invention relates to an objective, in particular an objective for a semiconductor lithography projection exposure machine, having a plurality of optical elements supported in mounts. The invention also relates to a method for producing an optical element.
The resists used in lithography are being increasingly loaded with higher pulse powers and pulse numbers of a laser. Resists generally consist of plastic and are changed upon the application of laser light, in which case they output emissions which are damaging to the optical elements of the objective used in the semiconductor lithography projection exposure machine. This holds, in particular, for the optical terminal surfaces which in this way lose their capacity for use in the course of time.
It is therefore known to subject the objective to a specific overpressure, gas being introduced continuously for this purpose, in order to keep the optics clean at all times. This prevents contaminants from penetrating inwards from the outside, because a slight overpressure always prevails in the interior of the objective. On the side directed toward the resist, the objective is provided with a terminal plate which forms a protection against contaminants from the resist and simultaneously also seals the objective.
With increasing aperture and more stringent wavefront requirements of recent systems in lithography, the downstream plane-parallel terminal plates are becoming increasingly problematic. A relatively large aperture in conjunction with an unchanged image field necessarily increases the plate diameter and consequently, given the same thickness, the sagging due to gravity, for example. However, a specific amount of sagging worsens the wavefront substantially in the case of a higher aperture. Although a thicker terminal plate could remedy this, this not only increases the dead weight of the objective, but can moreover also lead to heating up and to image quality problems resulting therefrom.
Because of the abovenamed disadvantages in the use of a terminal plate, an attempt is made in part to dispense entirely with the terminal plate. This is no problem with reference to the optical design of a lithography objective. However, if the last optical element, be this a lens or else the terminal plate, becomes useless, for example because of contaminants from the resist, the entire objective must be sent back to the optical company or to the manufacturer, where a new lens must be produced and the entire optical system must be readjusted and tuned. The errors which, to be specific, are introduced by a new lens and/or terminal plate are generally so large that it is impossible to dispense with renewed tuning. The new lens and/or the terminal place necessarily have homogeneity and surface imperfections and, in the case of a lens, also an offset and centering errors, to which are added mechanical restraints imposed by the mount in which the lens is embedded.
It is therefore the object of the present invention to avoid the abovenamed disadvantages, in particular to create an objective in which the last optical element which is particularly at risk from damage and contaminants can easily be exchanged if required.
This object is achieved according to the invention by means of the features named in the characterizing part of claim 1.
By virtue of the fact that the last optical element is connected directly to the penultimate optical element in a mount-free fashion, no renewed tuning is required in the event of its exchange. This means it is no longer necessary to return it to the manufacturer. A person skilled in the art who has a suitable optical element in store can, if required, exchange the damaged last optical element directly in situ. This can be done, for example, without difficulty and in a short time in a clean environment in a clean room directly on the objective because of the exchangeable and/or detachable connection of the last optical element to the penultimate optical element.
When, in a very advantageous development of the invention, it is provided that the last optical element is constructed as a thin equidistant plate, any sort of setting or tuning work can then be eliminated, because optically it is no longer important how accurately the equidistant plate is seated to the side, or how accurately it is centered, since it is of the same thickness (curved or flat) at every point.
A further very advantageous refinement of the invention can consist in that the last optical element is connected to the penultimate optical element by wringing. A simple connection, mount-free and exchangeable, of the last optical element to the penultimate optical element is thereby achieved. In this case, the two optical elements are juxtaposed without a clearance, which is referred to in joining technology as xe2x80x9cwringingxe2x80x9d or xe2x80x9cphysical gluexe2x80x9d. In wringing, use is made of the atomic forces at very short distances.
When using a last optical element made from crystalline material, it is advantageous to apply an amorphous inorganic layer, for example silica glass, to the side facing the penultimate optical element. Although it is also possible in principle for an optical element made from crystalline material to be wrung, because the crystals such as calcium fluoride or potassium fluoride, have only a relatively slight long range action to the outside with reference to their atomic forces, the application of an amorphous inorganic layer, for example vapor deposition, has the effect that during this vapor deposition, the atoms of silicon oxide, for example, penetrate in the vacuum into the crystal structure, the result being better adhesion.
The last optical element can be provided as the known terminal plate or, instead of a separate terminal plate, so can the last optical element, specifically the last lens of the objective. In the latter case, this actually means that the last lens is now of bipartite construction, the outer part being very thin by comparison with the inner part in one refinement of the invention, and the mount-free connection is situated between the outer part and the inner part of the lens. In other words, the inner part of the lens is held in a known way in a mount, while the outer part of the lens is connected to the inner part of the lens in a mount-free and exchangeable fashion. In this case, the known terminal plate is omitted without replacement.
An advantageous method for producing the last optical element is described in claim 14.
A thin wafer or equidistant plate is generally provided as the last optical element. The production of such a thin wafer with an appropriate accuracy and/or with avoidance of material defects is, however, a problem and/or such a plate must be appropriately accurate so that it can be used in situ instead of the damaged optical element in the way according to the invention without changing the optical properties of the objective. This can also be achieved in a relatively simple way using the method according to the invention. This is rendered possible, in particular, by the processing on an adapter whose values are known, for which reason the average values of the wafer or plate to be processed on the adapter, which is later to be used as a new optical element, can be matched as appropriate to the requirements by processing the surfaces.